Joseph cells, assumed to be photoreceptive organs in amphioxus, were studied by electron microscopy as well as by a histofluorescence technique. Each Joseph cell is composed of three parts: the microvilli extending in the peripheral portion, the cell body, and a cytoplasmic process. Light and dark treatments affected the ultrastructure of the microvilli. In darkness, microvilli became more numerous, thinner, and more regularly arranged. A specific fluorescence, indicative of the presence of retinal-based proteins, appeared only in the dorsal wall of the neural tube where the Joseph cells are situated. These results suggest that the Joseph cells are in fact photosensory with the microvilli containing a rhodopsin-like substance.